Quote - Props that represent bounded volumes will reliably block IBL 100% because IBL will not leak through two polygons on the same line.

I get the same thing also.  Very unexpected (you see, assumptions are perilous).  Although, when foremost normals are facing away from the camera (as in, a clear line of sight to the camera), the renderer appears to do something very different from when normals are facing towards it, even though the Normals_Foward property is set for all materials.  This is probably not good.

edit: err, the very different thing when rendering back-facing normals is "go very slowly"

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